1. Field of the Invention
This invention relates to a liquid crystal composition. More specifically, the present invention relates to a ferroelectric liquid crystal composition which has reduced viscosity as a liquid crystal composition while maintaining a bookshelf layer structure.
2. Description of the Related Art
Data processing apparatuses such as word processors, personal computers, workstations, and so forth, have become wide spread. The progress of office automation, the down-sizing of apparatuses, and the importance of display devices as an interface with these data processing apparatuses continue to increase. With such a background, liquid crystal displays (LCDs) have become indispensable because they use little power and are compact, flat-panel displays.
The LCD which has been most widely used for computers, etc, is a super-twisted nematic (STN) type liquid crystal. The STN LCD can be driven at a relatively low voltage (about 20 V) and can effect a medium information content display of 640.times.480 pixels with a contrast ratio of about 10:1. Accordingly, the STN LCD can satisfy the basic display functions of a personal computer or a word processor. As described above, however, even data processors such as workstations have been converted to personal-use systems with the progress of down-sizing as described above. Under such circumstances, higher information contents and area, higher response speed, higher contrast ratio, wider viewing angle, and so forth, have become prerequisites for display devices. Since the STN directly drives the liquid crystal by a simple matrix panel construction, it is not very difficult to accomplish a large area display having a diagonal of 15 inches or more, but it is extremely difficult to increase its information content to 1,240.times.1,028 pixels (to about 1,000,000 pixels, though the number of pixels depends on the model) as required by workstations, etc. Even if the number of pixels can be increased, the contrast ratio and the response speed of the STN drop remarkably, and the display quality of the STN falls.
With the background described above, the following two methods have been developed and proposed to LCDs produce which can cope with the requirements of workstations. One method is the active matrix LCD in which non-linear active devices, such as thin film transistors, are formed at the points of intersection (pixels) of the matrix. Another is the ferroelectric LCD which employs direct driving by a simple matrix in the same way as the STN LCD.
The active matrix system can apply a voltage for driving the liquid crystal while securing a sufficient voltage margin for each pixel. Therefore, a high contrast ratio and a best response can be expected. However, it is extremely difficult to fabricate a high information content wide-area active matrix panel of the order of 1,240.times.1,028 pixels and with a diagonal of more than 15 inches for the following two reasons. In the case of a present large-scale panels (at least 6 inches), thin film transistors must be fabricated by the use of amorphous silicon (a-silicon) for the technical reasons (semiconductor fabrication technology for a large area). However, the mobility of electrons of a-silicon necessary for exciting the transistor is low and there is a limit to the increase of the area and the information contents. It is believed that a panel size of about 15 inches is the technical limit at present from the aspect of electron mobility. The other reason is the problem of producibility. In the 1,240.times.1,028 pixel class, the production yield of the display panel, for which zero defects are required, drops remarkably and hence, the production cost becomes so high that it cannot be comparable to the production of a cathode-ray tube (CRT) display.
On the other hand, since the ferroelectric liquid crystal display employs the simple matrix panel structure and can be of high information content, it can theoretically be applied to a display of the workstation class. To accomplish a practical ferroelectric liquid crystal display (FLCD), however, several problems are yet to be solved. A particularly critical problem is how to attain a large area and uniform orientation of liquid crystal moleculars and how to make the liquid crystal layer structure uniform. The conventional ferroelectric liquid crystal has defects such as zigzag orientation, and has a particularly large orientation variability in a large area display, so that the contrast ratio depends on pixel position. It has been clarified that the zigzag orientation defect (or zigzag defect) results from the layer structure of the liquid crystal (Hiji et al, Japanese Journal of Applied Physics, Vol. 27, No. 1, 1988, pp. L1-L4). In other words, the conventional ferroelectric liquid crystal material has the layer structure having a Chevron structure which is bent in a &lt;-shape near the center of the panel as shown in FIG. 2 of the accompanying drawings and causes the orientation defects such as the zigzag defect. In a bookshelf layer structure wherein the layer structure is arranged perpendicularly to the substrate as shown in FIG. 1, on the other hand, the liquid crystal molecules are uniformly oriented. To practically accomplish uniform orientation and to obtain a high contrast ratio using a ferroelectric liquid crystal which can effect, in principle, large capacity displays and which can be applied to a display of the workstation class, it is very important to obtain the bookshelf structure for the liquid crystal layer structure and to eliminate the zigzag defect. To accomplish the bookshelf layer structure, on the other hand, the liquid crystal material itself must be improved, and this bookshelf layer structure can be obtained by the use of a specific liquid crystal material. The inventors of the present invention have clarified that this bookshelf layer structure can be obtained by using naphthalene type liquid crystals (Mochizuki et al, Ferroelectrics, Vol. 122, 1991, pp. 37-51). However, the conventional naphthalene type liquid crystals generally involve the problems that their viscosity is high and their electro-optical response characteristics particularly in the low temperature range drops. For these reasons, the viscosity of the naphthalene type liquid crystals as the liquid crystal composition must be lowered while maintaining the bookshelf layer structure.